The present invention relates to casting apparatus for continuous or semi-continuous direct chill casting of metal (DC casting). In particular, the apparatus is used for casting billets of aluminum for milling purposes. The casting apparatus includes a casting die which has an open inlet for receiving a supply of molten metal and a cavity with an open outlet. At the outlet, means are provided for supplying water for direct cooling of the molten metal and for supplying gas or air for reducing the cooling effect of the water, at least during the start phase of the casting process.
By using various methods, it has been shown that reduced cooling during the start phase of DC casting of metal results in positive effects with regard to shrinkage, start cracks and surface quality. It is also likely that the reduced cooling has a positive effect with respect to other problems associated with casting large billets.
In a known method which is disclosed in U.S. Pat. No. 4,693,298, air is added to the water before it leaves the water outlet. The water and air mixture then passes along the circumference of the casting die opening. The direction of the inlet of air in relation to the water is approximately 90.degree. so that air bubbles are produced in the water flow, i.e. the air is mechanically mixed with the water in the water flow. By replacing some of the volume of water with air, the intention is to achieve a uniform skirt of water with less water than is normally required to maintain a uniform skirt of water and, by means of the air, to achieve an insulating effect. The addition of air will, however, increase the speed of the water and thus also the cooling effect of a given quantity of water as the cooling water passes through the stream phase on the surface of the cast billet. Any reduction of the cooling effect of the water, caused by adding air to the cooling water before it leaves the water outlet, is therefore limited. Moreover, the solution as shown in the above patent offers no opportunities for differentiated cooling, i.e. a different level of cooling for one area in relation to another area along the casting die.
In a similar known solution, which is described in U.S. Pat. No. 4,166,495, CO.sub.2 is added to the cooling water instead of air. When the water exits the water outlet in the casting die, very small bubbles of CO.sub.2 will be formed due to the pressure drop and the increase in temperature. The CO.sub.2 bubbles form a partial insulating layer between the cast billet and the cooling water so that the overall cooling area is reduced. This method produces roughly the same reduction of cooling effect as the first-described method, but is more expensive to use because CO.sub.2 is used as the additive gas. Also, CO.sub.2 requires additional pressure regulating equipment and mixing equipment in order to obtain the necessary pressure conditions for the process to work. As above in the first-described method, this method does not provide any opportunity for differentiated cooling along the casting die or regulation of the cooling effect.
Furthermore, an article published in Metal Progress (No. 2 of 1957, pages 70-74), described a method for reduced or aborted cooling in which air nozzles are positioned slightly below the casting die. When the cooling water flows down over the cast billet and when the water reaches the nozzles, the water is blown away from the billet so that the area of the billet below the air nozzles is not exposed to direct water cooling. Only the area of the billet above the nozzles is directly cooled by the water. This solution does not reduce the cooling during the start phase of the casting process, and therefore, the positive effects realized regarding shrinkage and surface quality are small or insignificant.